1. Field of the Invention
The present invention relates to a highly reliable antireflection film formed on a bismuth-substituted iron garnet single crystal which is used as a Faraday rotator applied to magneto-optic sensors and optical isolators.
2. Description of Related Art
In recent years, optical fiber communications and optical instrumentation have made remarkable progress. Semiconductor lasers are widely used as a signal source in the optical fiber communications and optical instrumentation. However, semiconductor lasers are disadvantageous in that the oscillation becomes unstable due to so-called reflected light return where the light is reflected by, for example, the end surface of the optical fiber back to the semiconductor laser. In order to solve this drawback, an optical isolator is usually provided on the light-exiting side of the semiconductor laser. The optical isolator blocks the reflected light return, thereby stabilizing oscillation. The respective components that constitute the optical isolator are provided with antireflection films in order to prevent reflection for least optical loss.
An optical isolator includes a polarizer, analyzer, Faraday rotator, and permanent magnet. The permanent magnet causes the Faraday rotator to magnetically saturate. The Faraday rotator plays a major role in the optical isolator and is usually formed of a bismuth-substituted rare-earth iron garnet single crystal film having a thickness in the range from several tens to 400 .mu.m, grown by the liquid phase epitaxial method (referred to as LPE method hereinafter). Such single crystals include (HoTbBi).sub.3 Fe.sub.5 O.sub.12 and (LuTbBi).sub.3 (FeAl).sub.5 O.sub.12.
Conventionally, an antireflection film is formed by combining a plurality of materials having different refractive indices according to the refractive index of the material to which the antireflection film is applied, thereby providing as low a reflective index as possible. Of course, in some cases, a single material having a certain refractive index may sufficiently function as an antireflection film.
It is preferable that an antireflection film designed by combining materials having different refractive indices provides as low a reflection index as possible not only at the intended wavelength (referred to as central wavelength hereinafter) but also over a relatively wide range of wavelengths centered about the central wavelength.
A wide range of low reflective index is important since minute variations in manufacturing conditions of antireflection film may well cause the central wavelength of the antireflection film to deviate by several tens of nanometers from what it is designed to be.
The reflective index of a bismuth-substituted iron garnet single crystal film ranges from 2.3 to 2.4 in a near infrared region of 0.8 to 1.6 .mu.m. Antireflection films used for this range of reflective index include a three-layer antireflection film comprising layers of SiO.sub.2, Al.sub.2 O.sub.3, and SiO.sub.2, formed on the single crystal film in this order from the atmosphere side, and a two-layer antireflection film comprising layers of SiO.sub.2 and Ta.sub.2 O.sub.5, formed on the single crystal film in this order from the atmosphere side. These antireflection films provide low reflective indices not only at the central wavelength but also over a wide range of wavelengths centered about the central wavelength.
High reliability of components is of primary importance in optical communications. Recently, optical communications has placed greater demands on the component reliability. For example, requirement of humidity resistance was conventionally 85.degree. C, 85% RH (Relative Humidity), and 1,000 hours but now 85.degree. C, 85% RH (Relative Humidity), and 5,000 hours. This severe requirement is very difficult to meet. Conventional antireflection films are not sufficient to fulfill the requirement and there is a need for the development of an improved antireflection film.